I've been playing a new free game on my cell phone recently called Castle Clash. I've also got my roommate addicted to it It took my quite a while to find the hero upgrade costs so here they are.
When you first get a Hero they are level 1 and can reach level 20. A Hero can have 9 stars, and presumably reach level 180. You get your heroes experience by running dungeons and battling other players (the way that costs gold).
When a Hero maxes out it's level it can be upgraded. When upgraded it will gain a star, raises the Hero's max level by 20, and increases the Hero's stats a little.
|Star||Gold price||Hero Badge price||"Flames" requirement||
^ Conquering Dungeon rooms gives you "flames."